This invention relates generally to injection molding and more particularly to a heated nozzle for edge gating having alternate bores to receive a thermocouple element to monitor the operating temperature.
It is well known to monitor the operating temperature by mounting a thermocouple element in a bore extending inwardly near the front end of a heated nozzle. Examples are shown in the applicants' U.S. Pat. Nos. 4,768,283 which issued Sep. 6, 1988, 4,981,431 which issued Jan. 1, 1991, and the applicants' Canadian Patent application Ser. No. 2,101,480 filed Jul. 28, 1993 entitled "Injection Molding Nozzle which Retains a Thermocouple Element." U.S. Pat. No. 4,981,431 shows an edge gated configuration with the thermocouple element bore extending inwardly between the front end of the heating element and the seals extending to the gates. While these previous configurations are satisfactory for many applications, there is a problem that there is a temperature gradient near the front end of the nozzle due to heat loss to the surrounding cooled mold. This is particularly true for edge gating where the front end of the heating element must be spaced far enough from the front end of the nozzle to leave room for the outwardly extending seals. Thus, the accuracy of the measurement of the operating temperature is very dependent upon where the thermocouple element receiving bore is located between the heating element and the front end of the nozzle. The temperature measured by the thermocouple element relative to a temperature setpoint determines how much heat is provided by the heating elements. Thus, erroneous measurement of the operating temperature results in various problems dependent upon the type of material or melt being processed. For instance, if a crystalline material with a narrow temperature window or span between the melting point and processing temperature is being processed and the thermocouple element receiving bore is very close to the heating element, the temperature setpoint has to be well above the operating temperature prescribed for the material to provide sufficient heat to avoid the melt in the melt passage near the gates dropping below the solidification temperature. Conversely, if an amorphous material is being processed and the thermocouple element receiving bore is closer to the front end of the nozzle, the temperature setpoint can be well below the operating temperature prescribed for the material to avoid melt degradation, stringing and slower cycle time due to excessive melt temperatures. This problem of erroneous temperature measurement due to heat loss is exacerbated by the fact that the melt itself generally is a considerably better insulator if it is amorphous rather than crystalline. While an operator can compensate for this problem by adjusting the temperature setpoint higher or lower than the prescribed operating temperature dependent upon the type of material and location of the thermocouple element receiving bore, this has been found to be very confusing due to the fact that a material can only be processed with a temperature setpoint which is outside of the recommended processing temperature range for that material.